In order to understand better if and where erosion rates calculated using
in situ 10Be are affected by contemporary changes in land use and
attendant deep regolith erosion, we calculated erosion rates using
measurements of in situ 10Be in quartz from 52 samples of river sediment
collected from three tributaries of the Mekong River (median basin area = 46.5 km2). Erosion rates range from 12 to 209 mm kyr−1 with an
area-weighted mean of 117 ± 49 mm kyr−1 (1 standard deviation) and median
of 74 mm kyr−1. We observed a decrease in the relative influence of human
activity from our steepest and least altered watershed in the north to the
most heavily altered landscapes in the south. In the areas of the landscape
least disturbed by humans, erosion rates correlate best with measures of
topographic steepness. In the most heavily altered landscapes, measures of
modern land use correlate with 10Be-estimated erosion rates but
topographic steepness parameters cease to correlate with erosion rates. We
conclude that, in some small watersheds with high rates and intensity of agricultural land use that we sampled, tillage and resultant erosion has
excavated deeply enough into the regolith to deliver subsurface sediment to
streams and thus raise apparent in situ 10Be-derived erosion rates by as
much as 2.5 times over background rates had the watersheds not been
disturbed.